The present invention relates to a method for operating an exhaust gas treatment system, comprising an SCR catalytic converter for cleaning an exhaust gas of a motor vehicle internal combustion engine, wherein an ammonia-containing reducing agent is dosed to the exhaust gas and an exhaust enriched with NH3 according to the dosing is fed to the SCR catalytic converter, wherein an ammonia filling level value for a filling level of ammonia stored in the SCR catalytic converter is calculated by a computer model.
For the catalytically supported removal of nitrogen oxides (NOx) from internal combustion engine exhaust gases, it is known to add aqueous urea solution as a reducing agent containing ammonia (NH3) to the exhaust gas. In the hot exhaust gas, NH3 is released as the actual selective reducing agent with regard to the NOx reduction at a so-called SCR catalytic converter by means of thermolysis and/or hydrolysis. Thus, one problem is determining an appropriate feed rate for the reducing agent while avoiding an overdosing from an undesired slip of NH3.
For solving these problems, EP 1 348 477 A1 discloses determining a usage rate of NH3 stored in the catalytic converter and a NOx cleaning rate and to control the reducing agent feed in dependence on these variables in connection with a filling level of NH3 stored in the catalytic converter.
Published application EP 0 554 766 A1 discloses a method for the NOx reduction in exhaust gases by controlled hyperstoichiometric addition of NH3, wherein NH3 is dosed upstream of a catalytic converter until the NH3 amount stored in the catalytic converter has reached an upper threshold value. The NH3 stored in the catalytic converter is used with the NOx emitted by the internal combustion engine and fed to the catalytic converter, whereby the toxic substance NOx is removed from the exhaust gas. The NH3 feed is again carried on when the NH3 amount stored in the catalytic converter has reached a lower threshold.
In these and other known similar methods, a model-based combination of measurement values and stored characteristic values often takes place, for example for the SCR catalytic converter. A NH3 feed rate and a modeled usage rate are balanced by an integration continuously carried out and the reducing agent dosing rate is chosen in such a manner that a NH3 filling level that is aimed for results in a desired catalytic converter efficiency. It has, however, been shown that errors caused by, for example, parameter changes add up in the course of time, so that the correspondence between the modeled NH3 filling level of the catalytic converter and the actual value worsens and dosing errors result in the form of an underdosing or overdosing. The result of this is a reduced cleaning effect or an increased NH3 slip.
Exemplary embodiments of the present invention provide a method for operating an exhaust gas cleaning system with an SCR catalytic converter, which provides an improved nitrogen oxide conversion.
An exemplary method for operating an exhaust gas treatment system, comprising an SCR catalytic converter for cleaning an exhaust gas of a motor vehicle internal combustion engine is provided. The method involves adding, by a dosing unit controllable by a control unit, an ammonia-containing reducing agent to the exhaust gas at an adjustable dosing rate; feeding an exhaust gas enriched with ammonia according to the dosing is to the SCR catalytic converter; determining, using a computer model, an ammonia filling level value for a filling state of ammonia stored in the SCR catalytic converter; determining, using the computer model, a model dosing rate for the dosing of the reducing agent into the exhaust gas, at which a target filling level of ammonia stored in the SCR catalytic converter specified by the computer model or a target efficiency specified by the computer model for a nitrogen oxide conversion with ammonia stored in the SCR catalytic converter or fed to the SCR catalytic converter must be at least approximately achieved; performing either a model-based filling level regulation for achieving the target filling level or a model-based efficiency control for achieving the target efficiency, wherein, with an active filling state regulation and when presettable values are exceeded, an efficiency control is switched to when exceeding presettable values for
a current filling capacity determined by the computer model for an ammonia amount that can maximally currently be stored in the SCR catalytic converter,
a temperature of the exhaust gas or of the SCR catalytic converter,
a mass flow of exhaust gas flowing into the SCR catalytic converter, or
a mass flow of nitrogen oxide flowing into the SCR catalytic converter,
wherein a base efficiency is determined by the computer model that at least approximately corresponds to a currently possible maximum efficiency for the NOx conversion of the SCR catalytic converter and a presettable fraction of the base efficiency is given as the target efficiency with an active efficiency control.
With the method according to the invention, a NH3 filling level value for a filling level of ammonia stored in the SCR catalytic converter is calculated. A model dosing rate for the dosing of the reducing agent into the exhaust gas is calculated by the computer model, with which a target filling level of NH3 stored in the SCR catalytic converter presettable by the computer model or a presettable target efficiency for a NOx conversion with NH3 stored in the SCR catalytic converter and/or fed to the catalytic converter shall be reached at least approximately. A dosing unit is thereby accessed for the emission of the reducing agent with a corresponding dosing rate.
It is provided according to the invention to perform either a model-based filling level regulation for reaching the target filling level or a model-based efficiency for reaching the target efficiency. A switch-over from the filling level regulation to the efficiency control and vice versa is carried out depending on presettable values for a current filling capacity determined by the computer model for an ammonia amount that can maximally currently be stored in the SCR catalytic converter and/or for a temperature of the exhaust gas or of the SCR catalytic converter, and/or for a mass flow of exhaust gas flowing into the SCR catalytic converter, and/or for a mass flow of nitrogen oxide flowing into the SCR catalytic converter. The filling level of NH3 stored in the SCR catalytic converter is important for the NOx conversion of the SCR catalytic converter in wide operating regions. Using the model-based filling level regulation, a high efficiency for the NOx conversion of the SCR catalytic converter can be reached at least approximately in wide operating regions. The use of a computer model thereby allows for the fact that the NH3 filling level of the SCR catalytic converter is not accessible for a direct measurement or only in an inexact manner with considerable effort. A model-based filling level regulation is thereby meant to be a regulation method where a NH3 filling level of the SCR catalytic converter determined by the computer model is regulated as a regulation variable with a return to the target filling level which is also presettable by the computer model. An efficiency of the NOx conversion is meant to be a relative NOx conversion which results as a quotient of an NOx amount converted to non-toxic components during a time interval and an NOx amount flowing into the SCR catalytic converter in the same time interval.
It has, however, been shown that it is difficult to realize an NOx conversion free from NH3 slip with the desired exactness or to the desired extent using the of the model-based filling level regulation. This problem is faced by the switch to the model-based efficiency control for reaching the target efficiency provided according to the invention. A model-based efficiency control is thereby meant to be a control of the efficiency for the NOx conversion of the SCR catalytic converter, wherein the target efficiency to be reached is adjusted at least approximately by the computer model while foregoing the return of a determined actual efficiency in a controlled manner.
Presettable values for the current NH3 filling capacity of the SCR catalytic converter thereby serve as switch-over criteria from the filling level regulation to the efficiency control, and/or for a temperature of the exhaust gas or of the SCR catalytic converter, and/or for a mass flow of exhaust gas flowing into the SCR catalytic converter, and/or for a mass flow of nitrogen oxide flowing into the SCR catalytic converter. The respective variables can be determined indirectly from operating data of the internal combustion engine using the computer model or by a direct measurement. Using the switch-over between the filling level regulation and the efficiency control provided according to the invention, altogether a high NOx conversion free of NH3 slip is altogether achieved in an advantageous manner in very wide operating region limits and thereby altogether a very low NOx emission.
In one arrangement of the invention, a base efficiency is determined by the computer model, which base efficiency corresponds at least approximately to a currently possible maximum efficiency for the NOx conversion of the SCR catalytic converter and has a presettable fraction of the base efficiency given as target efficiency with an active efficiency control. With an active efficiency control, control is to a target efficiency that is only slightly below the maximum possible efficiency. A control to a target efficiency of about 95% to 70% of the maximum possible efficiency determined by the computer model. In this manner, a sufficiently high NOx can be achieved on the one hand even while foregoing a regulation, on the other hand, an NH3 slip, which can result with a higher efficiency, can be avoided with a high safety.
In a further arrangement of the invention, a pre-control of the dosing rate is carried out with an active filling level regulation and/or with an active efficiency control. A value for the dosing rate of the reducing agent is determined from the computer model, with which the target filling level or the target efficiency should result corresponding to the deposited model. This value serves as a pre-control value, with which the dosing unit is controlled. In the case of the active filling level regulation, resulting deviations thus only have to be regulated. Thus, the reference action of the control loop is improved. In the case of the active efficiency control, the NOx content in the exhaust gas can be sensed on the output side of the SCR catalytic converter using an NOx sensor and it is compared by the computer model if the measured NOx content at least approximately coincides with the NOx content resulting from the target efficiency. In the case of a deviation that is not tolerated, the adjusted dosing rate is changed correspondingly.
In a further arrangement of the invention, a correction to a target dosing rate by a changeable long term adaption factor is provided for the model dosing rate with the active filling level regulation and/or with an active efficiency control and in a further arrangement of the invention, a correction to an assumed actual filling level is provided by a changeable short term adaption factor by an active filling level regulation for the ammonia filling level value. Thus, the possibility is provided according to the invention to perform a correction acting directly on the model dosing rate, which will become immediately effective on the one hand, but is possibly however maintained over a longer time on the other hand. On the other hand, the possibility is provided to perform a correction of the modeled NH3 filling level acting in the short term. In this manner, disturbing influences occurring in the short term and disturbing influences effective over a longer time, for example gradually progressing ageing or drift effects, can be compensated for.
In a further arrangement of the invention, nitrogen oxide emission values and/or ammonia emission values of the exhaust gas are continuously determined, and the nitrogen oxide and/or ammonia emission values are evaluated cyclically with regard to fulfilling the presettable short term adaption criterion and/or a long term adaption criterion. The nitrogen oxide and/or ammonia emission values of the exhaust gas are preferably determined by values sensed in a measurement-technical manner using a suitable exhaust gas sensor. The emission values can, however, also be taken from operating characteristic fields. In this manner, a direct control of the filling state regulation or of the efficiency control is enabled with regard to a correct operation. A modeled NH3 filling level value, possibly corrected subsequent to a cycle to the assumed actual filling level, then serves as the current filling level value, on which a further balancing integration and dosing sits in order to achieve the target filling level or the target efficiency. This is synonymous with a short term adaption of the Nh3 filling level value, with which is reacted promptly to short term influences, which possibly lead to a false modeling of the NH3 filling level. However, a correction of the model dosing rate, possibly performed by multiplication with the long term adaption factor, has an effect beyond a cycle, which is synonymous with a long term adaption. By the procedure according to the invention with a cyclically performed check of modeled operating variables, a cyclic adaptation of these operating variables is enabled if this is necessary.
Advantageous embodiments of the invention are illustrated in the drawings and are described in the following. The previously mentioned characteristics and which will still be explained in the following cannot only be used in the respectively given combination but also in other combinations or on their own without leaving the scope of the present invention.